Perhaps one of the most complex engineering tasks to be undertaken in any major tunnelling project is managing the logistics of delivery and installation of the tunnel boring machine (TBM) to the drive site. With smaller diameter TBMs, it is often possible to deliver the shield whole or in larger assembled sections. This is not usually possible for larger-diameter TBMs as the size and weights of the various major components (cutterhead, main bearing, erector, shield sections, etc.) can make it impractical or impossible to deliver the shield wholly or even partially assembled.
However, owing to the Chambers Wharf site being on the River Thames and located downstream of Tower Bridge, the opportunity to deliver the TBM shield in its entirety was not only possible, but it was also practical and in-line with the Tideway strategy of maximising delivery by river. After factory assembly and testing, the shield and back-up was relocated from Schwanau, Germany to the port of Rotterdam, Holland and was ready to be loaded easily onto a sea- and river-going vessel for delivery to the UK.
This was to be a unique challenge however: how to safely and efficiently design, procure and execute the delivery of nearly 900t of TBM from Rotterdam, the busiest port in Europe, across the North Sea – one of the world’s busiest for marine traffic – to Central London, which has one of the world’s most iconic and navigable of rivers.
DELIVERY LOGISTICS
Herrenknecht, the TBM supplier, had a close working relationship with Mammoet, a firm that is renowned internationally for being able to lift and shift large plant and material. As such, Mammoet was the natural choice to execute delivery and lowering of the TBM into the shaft. However, to be able to carry it out, the delivery would require the development of a number of bespoke engineering solutions, detailed planning, and collaboration with the river regulators, as well as engagement with a multitude of stakeholders.
Delivery logistics had been embedded in the design of various elements of the site: the height of the flood defence was always to be maintained; the gantry crane and acoustic enclosure structural envelope was to allow delivery either under or over it, in addition to its longer-term purpose; and the main door of the acoustic enclosure was designed to accommodate the machine’s delivery in multiple ways.
Mammoet and CVB (the JV comprising Costain, Vinci Construction Grand Projets and Bachy Soletanche) developed two delivery concepts:
1. Using a standard barge to deliver the TBM to site; lifting it onto the site with an 1800t-capacity marine crane, lifted over the site gantry-crane structural envelope and strand-jacked into the shaft, or
2. Delivery on a 3000t jack-up barge with a direct drive-off via self-propelled modular transport (SPMT) onto the site, with sufficient height under the site gantry-crane’s structural envelope, then strand-jacked into the shaft
These allowed shield and gantries one and two (which were the minimum necessary) to commence tunnelling operations:
Both methods were put into value engineering assessment: the first option was technically the more favourable as it required less of an engineering solution to be developed for the site. But ultimately, it was not chosen, as the size of the vessel and the disruption its operations would cause to river traffic were not acceptable to the Port of London Authority (PLA).
As such, delivery using the Ravestein BV Skylift 3000 was put into detailed planning and design development. Notable engineering challenges that needed assessment and solution development were:
1. Design of the passage from Rotterdam to Chambers Wharf, including considerations of the TBM sea fastenings, tidal performance, the weather along the route with a barge and Skylift measuring 28m wide and 58m-high passing under London City Airport take-off and landing paths, through the Thames Barrier and beneath the Emirate Skyline cable car.
2. Detailed design of the Skylift arrival, mooring and jack-up against the cofferdam, with special consideration to the position and height of the gantry crane structure and the combined effects of tidal flows and weather.
3. Assessment of the cofferdam performance with the Skylift ‘spud’ legs driven into the riverbed and then subsequent performance post removal (including embedment depths, possible settlement and the need to backfill the penetration post leg removal),
4. Unexploded ordnance (UXO) and archaeological surveys to confirm spud locations were not at risk of encountering UXO or missed archaeological items.
5. Assessment of the roll-on roll-off (Ro-Ro) ramp from the Skylift deck to the cofferdam and its bearing loads against the cofferdam’s bearing capacity, and the on-deck performance of the barge,
6. Assessment of the floating and jack-up levels of the Skylift to permit sufficient clearance to the underside of the gantry crane structure, but also permit allowable height and Ro-Ro ramp grade.
7. Design, installation and operation of a ‘flood defence gate’ which was to maintain the cofferdam flood defence height but also permit the Ro-Ro ramp to be installed and used.
8. Bearing capacity of the cofferdam for the TBM shield, cradle and SPMT loadings for drive-off, and transition to the installation position.
9. Structural assessment of the cofferdam, shaft capping beam and shaft permanent secondary lining to sustain the strand-jacking system loadings during the lowering operation.
Detailed planning and engineering assessments for the delivery commenced in late autumn of 2019 between Mammoet, CVB and temporary works checker Wentworth House Partnership (WHP), along with close engagement and collaboration with river regulators the Environment Agency (EA); the Marine Management Organisation (MMO); and the PLA. Most temporary works checks came to a simple completion, however one key engineering issue that was prevalent with WHP and all three regulators was cofferdam assessment and spud-leg penetration into the riverbed.
REGULATORS’ CONCERNS
The concerns of regulators and WHP included:
1. Performance of the cofferdam with the spud leg installed into the riverbed:
a. To what depth they would penetrate based on design and/or riverbed make-up, and how that would effect the cofferdam bearing capacity?
b. Would the leg penetrate deep enough to create sufficient bearing capacity for barge stability; how was this depth determined and what allowance was there for error?
c. If the legs penetrated deeper, could it cause a pathway to an underground aquifer leading to an environmental issue?
2. Performance of the cofferdam once the spud leg was removed from the riverbed:
a. What would the effects be of backfilling the penetrations or leaving them to naturally close in with tidal movements?
b. What materials would be suitable for the backfilling and what pollution might the backfill cause?
c. How would CVB guarantee that backfilling would be sufficient, correctly placed and would not cause obstruction to marine traffic?
All these concerns were valid, and the performance of the spud legs and cofferdam were easily assessed as suitable. However, there was no consensus between regulators as to whether to backfill, how much to backfill or what backfill material should be used. Reaching a final solution that was agreeable to all three regulators required negotiation and further design input from WHP, with the eventual solution being to backfill the two penetrations closest to the cofferdam with washed, single-size river aggregate along with pre- and post-bathometric surveys to confirm levels and lack of obstruction. In early summer 2020, consent for the delivery was agreed to by the regulators.
TIMELINE
The Skylift departed Rotterdam on 7 July and arrived at Coldharbour Jetty at 12:30 on 8 July. The SkyLift then departed at 14:30 and sailed up the River Thames, arriving at Chambers Wharf just after 17:00 where it was temporarily moored. On 10 July, the Skylift was repositioned and jacked up into the offload position and removal of the sea fastenings commenced. At 14:15 on 11 July, TBM Selina was driven over the Ro-Ro ramp and onto the Chambers Wharf Cofferdam where it was temporarily stored until the shaft and launch adit were ready for lowering and installation. When the Skylift eventually departed, the penetrations were surveyed and backfilled later that day.
Delivery of TBM shield and gantries one and two was always planned for late June / early July. However, due to the coronavirus pandemic and national lockdown, works at Chambers Wharf to prepare the site for the TBM’s arrival and lowering were postponed, followed by the implementation of Covid control measures during the subsequent slow return to work. Lowering of the TBM was then postponed until the middle of August while the SCL activities to prepare the launch adit were being completed.
Detailed planning and engineering assessments for the lowering operation using the strand-jacking system were undertaken. This occurred in parallel with the delivery, and required a review of the systems as well as the bearing capacity of the shaft lining, capping beam and cofferdam to support the operation. During the planning, it became apparent that an outstanding non-conformance report (NCR) relating to the secondary lining works was going to have a significant impact on the system and its operations. The setup of the strand-jacking system meant the vast majority of the loadings were to be placed on the newly-installed permanent secondary lining.
The shaft secondary lining was constructed using double-sided slip formwork that was raised up the shaft. This method produced an annulus gap between the shaft primary lining and secondary lining that was to be filled with structural grade concrete to provide confinement. During the final stages of this work, a substantial non-conformance occurred which meant the annulus could not be filled to the top of the secondary lining and was therefore terminated 4m lower than the top of the shaft lining; this meant the original design assumptions for the strand-jacking bearing were no longer valid.
The investigation and subsequent review of potential remedial actions had begun, however the programme for lowering the machine was now at risk. Therefore, a decision was taken to halt the remedial works in relation to the NCR and implement a review of the now unconfined secondary lining to sustain the strand-jacking system loadings.
1. Detailed investigation and analysis were conducted between Mott Macdonald, OTB consultants and CVB, and a new temporary works solution was devised to resolve the situation promptly. The solution involved a steel stool being fabricated and placed in fixed bearing locations, along with an anti-bursting dowel that was drilled into the lining below to sustain the temporary loading. This solution proved to be quite effective and easily installed, mitigating any programme risk and allowing the strand-jacking system to be installed on programme.
Mammoet was to mobilise again to Chambers Wharf on 7 August 2020, with the machine being successfully lowered on 28 August, and Mammoet demobilising immediately after.
